33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 237 



Simple mutations of many temperate phages are known which result 

 in an extreme reduction of the ability to enter the prophage state. Cells 

 infected by such mutants are almost invariably lysed. Such mutants are 

 called "clear," as lambda^ ear (Ac). 



Another more complex class of mutants of temperate phages is known, 

 the members of which invariably lyse the infected cells. These mutants 

 can also overcome the immunity conferred upon a cell by the presence 

 of the prophage from which they were derived. Such mutants are called 

 "virulent," as lambda V i r (Avir). 



Relatively little is known as yet of the biochemistry of lysogenization 

 or of induction and its consequent events. Even the nature of prophage, 

 which is almost invariably assumed to be DNA, can scarcely be regarded 

 as convincingly demonstrated. 



2. Structural Aspects of Some Temperate Phages 



Only "preliminary" analyses are available of the DNA of only three 

 species of temperate bacteriophage (Table I). Since there are reasons to 

 believe that the DNA of such phages bears some genetic homology to 

 the DNA of their host cells, 6, 7 it is perhaps to be expected that these 

 phage contain the usual deoxyribonucleotides found in the host. Whether 

 the obvious deviations of the data available from the usual adenine-thy- 

 mine and guanine-cytosine complementarity are to be attributed to the 

 preliminary character of the data or represent a more fundamental fea- 

 ture remains to be decided. 



The most detailed studies of the properties of temperate phage and the 

 lysogenic state have been made with four species which attack various 

 strains of E. coli and Shigella: lambda (X), PI, P2, and P22. 7 Data con- 

 cerning the composition and morphology of these phages are included in 

 Tables I and IV. 



3. Biochemistry of Lysogenization 



The fraction of cells lysed as compared to the fraction made lysogenic 

 by a particular temperate phage depends strongly upon the culture condi- 

 tions including the medium, the age of the cells, the temperature, etc. 

 In general, processes which lead to an inhibition of protein synthesis favor 

 the establishment of the lysogenic state. 189, 190 Such inhibiting agents (chlor- 

 amphenicol, 5-hydroxy uridine, amino acid analogs) are more effective if 

 given some minutes after infection. It is believed that this delay permits 

 vegetative replication of the phage; inhibition of protein synthesis then 



187 H. Ionescu, Compt. rend. acad. set. 237, 1794 (1953). 



188 N. D. Zinder, Virology 5, 291 (1958). 



189 J. R. Christiansen, Virology 4, 184 (1957). 



190 L. E. Bertani, Virology 4, 53 (1957). 



